Extending Polymer Flooding Towards High-Temperature and High-Salinity Carbonate Reservoirs

Abstract

Polymer flooding is a mature EOR technique successfully applied in both sandstone and carbonate reservoirs. ADNOC has developed a new EOR roadmap with the objective to identify and mature EOR options to improve displacement and sweep efficiency in carbonate reservoirs. Polymer based EOR was identified as one of the main EOR options. These options include polymer injection, simultaneous injection of miscible gas and polymer (SIMGAP), simultaneous injection of water and polymer (SIWAP), low salinity polymer, etc. However, the conditions of the reservoirs in Abu Dhabi are beyond the industry experience for the application of polymer which pose a significant challenge to polymer based EOR processes. The reservoirs are at high temperature (~100-130 °C), the formation brine is of high salinity (~200,000 ppm), the brine also has high concentrations of divalent ions (~18,000 ppm of Ca++ and Mg++) and the reservoir formation is carbonate where there is little experience in the industry for polymer injection. The stability of polymers is known to be severely affected at such conditions of high temperature, salinity and divalent ions concentration. Therefore, the main challenge for polymer based EOR processes in ADNOC reservoirs (such as polymer flooding, SIMGAP, SIWAP, etc.) is to find a polymer that are stable under such extreme conditions and can be injected in carbonate reservoirs. ADNOC has lunched a number of studies to experimentally investigate 1- the stability of polymers at such adverse conditions of high salinity, high temperature and high divalent ions, 2- injectivity of polymers in carbonate rocks using both outcrop and reservoir core plugs, 3- bulk and in-situ rheology of polymers, 4- effects of various parameters on the polymer performance, such as polymer concentration, shear rate and presence of oil on polymer retention and in-situ rheology and 5- impact of H2S and Oxygen on polymer stability. In this paper, we report the results of these different studies that can enlarge the application envelope of polymer flooding to high-temperature, high-salinity and light oil Middle Eastern carbonate reservoirs. The main conclusions of the studies are: 1- A polyacrylamide based polymer with high content of ATBS (SAV 10) was identified as stable at Abu Dhabi reservoir conditions, 2- SAV 10 polymer is also stable in the presence of H2S (500 ppm) and/or Oxygen up to 150 ppb, 3- The polymer has good injectivity in a wide range of injection rates ranging from 1ft/day to 120ft/day and wide range of permeability, 4- The polymer showed a shear thickening behavior with an increase in flux without any signs of mechanical degradation, noted by the stable viscosities of the effluents, 6- The presence of crude oil had significant impact on injectivity, in-situ rheology and adsorption in carbonate core material. In summary, the ATBS polymer showed a promising injectivity behavior which can be modulated for injection in the target reservoirs. In addition, as the required viscosity increase for both SIMGAP and SIWAP processes to work is moderate, we find the results to be very promising and they open the door for field testing and piloting.